TiO2/MWCNT/Nafion-Modified Glassy Carbon Electrode as a Sensitive Voltammetric Sensor for the Determination of Hydrogen Peroxide

In this work we describe a straightforward approach for creating a nanocomposite comprising multiwalled carbon nanotubes (MWCNTs) and titanium dioxide (TiO2) using the hydrothermal technique, which is then characterized by scanning electron microscope (SEM), energy-dispersive X-ray spectrometer (EDS), X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA) to assess its properties. Nafion is employed as a reticular agent for the nanocomposite on the glassy carbon electrode (GCE), creating the MWCNT/TiO2/Nafion/GCE system. The electrochemical behavior of the system was evaluated using cyclic voltammetry, revealing its remarkable electrocatalytic activity for detecting hydrogen peroxide in water. The developed sensor showcased a broad linear response range of 14.00 to 120.00 μM, with a low detection limit of 4.00 μM. This electrochemical sensor provides a simple and highly sensitive method for detecting hydrogen peroxide in aqueous solutions and shows promising potential for various real-world applications, particularly in H2O2 monitoring.


Introduction
Hydrogen peroxide (H 2 O 2 ) is a simple but significant compound in various applications, such as pharmaceuticals, clinical, environmental, mining, textiles, and food manufacturing.H 2 O 2 is a signaling molecule regulating essential biological processes, such as immune cell activation, vascular remodeling, and apoptosis.H 2 O 2 is a secondary product of various enzymatic reactions [1][2][3][4].Due to its importance as a regulating molecule, hydrogen peroxide is an excellent model molecule for applications in developing new electrochemical sensors.
Electrochemical methods are the most promising for detecting H 2 O 2 because of their advantages, such as easy miniaturization, rapid response, simple instrumentation, and high specificity and sensitivity.The natural concentration of H 2 O 2 varies from micromolar (µM) to tens of millimolar (mM) [5].While various techniques exist for H 2 O 2 determination, such as chromatography [6], chemiluminescence [7], colorimetry [8], and titrimetric analysis [8], these methods are often complex, expensive, and time consuming.In recent years, many electrochemical approaches have been developed to determine low concentrations of H 2 O 2 in a high-throughput fashion in fast, simple, reliable, and inexpensive ways [8][9][10].
Electrochemical sensors are suitable for various matrices, require minimal sample preparation, and exhibit high sensitivity and a wide concentration range with low limits of detection [11,12].Enzymatic and non-enzymatic sensors have been developed for detecting H 2 O 2 [13,14].Enzymatic approaches exhibit excellent selectivity and sensitivity but lack stability, require complex and expensive immobilization processes, and are highly dependent Sensors 2023, 23, 7732 2 of 14 on experimental conditions.Therefore, developing non-enzymatic electrochemical H 2 O 2 sensors is very interesting for various biomedical, industrial, and academic applications.Non-enzymatic electrochemical sensors have been developed by chemically modifying electrodes with nanomaterials, such as nanoparticles of noble metals, transition metals, metallic oxides/hydroxides, bimetallics/alloys, and carbon nanomaterials (CNT, graphene and its compounds) [5,15,16].
Among the promising metals for developing nanomaterials, titanium dioxide stands out.Titanium dioxide (TiO 2 ) is a transition metal oxide belonging to the category of ntype semiconductors, with its bandgap energy ranging from 2.9-3.2eV [17], depending on the crystal phase [18].Due to its high electrochemical activity, excellent mechanical and chemical stability, and exceptional capacity for organic molecule adsorption, TiO 2 is widely used in electrochemical sensors [19,20].However, to further improve the sensing performance of TiO 2 , researchers are working on forming a hybrid structure with different materials with diverse functional properties.One such material extensively studied in electrochemical applications is multi-walled carbon nanotubes (MWCNTs), which possess remarkable physical and chemical properties, such as high electrical conductivity, large surface area, and high energy storage capacity [21].Previous investigations have shown that modifying electrodes with MWCNTs substantially enhances the rates of electron and proton transfer, resulting in superior peak separation and enhanced electrode sensitivity compared to other types of modified electrodes [22,23].Additionally, the anchoring of semiconductors' metal oxide particles onto MWCNTs with homogenous distribution has exhibited improved electronic properties, making it a promising platform for selective sensing and catalytic processes.
Among various nanocomposites, the TiO 2 /MWCNTs nanostructures [24] and other notable composites, such as TiO 2 /MWCNTs/Pt [25][26][27], Au@TiO 2 /MWCNT [28], ZnS/Au10/f-MWCNT [29], and PB-TiO 2 /CNT [30], have been reported to exhibit exceptional electrocatalytic activity toward hydrogen peroxide (H 2 O 2 ) and other analytes in biosensing applications.However, it should be mentioned that the growth of complex nanostructures and deposition of catalytic nanoparticles, as previously demonstrated in the literature, typically requires sophisticated techniques such as electrochemical anodization, electrochemical deposition, or photo-induced deposition.Although these techniques have enabled the creation of intricate nanostructures, they can be both time consuming and resource intensive [31].
In this context, the sol-gel technique combined with hydrothermal synthesis has emerged as a promising approach for the preparation of nanometer-sized particles and materials with high specific surface area [32].TiO 2 /MWCNT nanocomposites can be prepared using this technique, which shows potential for developing chemical and electrochemical sensors or promoting novel applications in biosensing [33] and photodetection [33] thanks to its unique three-dimensional network texture, high surface area, and exceptional electrocatalytic activity.
This study presents a novel sensor for detecting H 2 O 2 that employs a TiO 2 /MWCNT/ Nafion nanocomposite as a surface modifier for a glassy carbon electrode (GCE) in conjunction with cyclic voltammetry (CV).The sensor exhibits high sensitivity and a low detection potential for H 2 O 2 .Notably, this is the first report on the use of TiO 2 /MWCNT/Nafion on GCE for electrochemical H 2 O 2 detection in water using CV.Our findings indicate that the TiO 2 /MWCNT/Nafion nanocomposite holds promise for developing high-performance H 2 O 2 sensors with a facile synthesis process, making it a viable candidate for creating sensors for various clinical and environmental applications.

Synthesis of TiO 2 Particles
The TiO 2 particles were prepared with modifications by the sol-gel method proposed by Ferreira-Neto, Elias et al. [20], represented in Figure 1.Initially, 750.00 µL of titanium isopropoxide (TiP) was slowly added to 100.00 mL of a mixture composed of ethanolisopropanol [3:1 (v/v)] under magnetic stirring, which was maintained for three hours.
8-25 nm and length ranging from 5.00 to 30.00 μm and a purity of ≥93% were provided by the Nanomaterials Laboratory of the Department of Physics, UFMG.Nafion-117 was purchased from Sigma-Aldrich (Oakville, ON, USA).

Synthesis of TiO2 Particles
The TiO2 particles were prepared with modifications by the sol-gel method proposed by Ferreira-Neto, Elias et al. [20], represented in Figure 1.Initially, 750.00 μL of titanium isopropoxide (TiP) was slowly added to 100.00 mL of a mixture composed of ethanolisopropanol [3:1 (v/v)] under magnetic stirring, which was maintained for three hours.
Subsequently, 9.00 mL of a mixture of deionized water and solvent (3.00 mL H2O:6.00 mL of the ethanol-isopropanol solvent) were added dropwise to form TiO2 particles through hydrolysis and condensation of titanium alkoxide species.After 2 h of magnetic stirring, the resulting colloidal suspension was centrifuged at 3500 rpm for 5 min and washed once with the solvent used in the reaction.
To crystallize the amorphous TiO2, the obtained material was resuspended in 32.00 mL of deionized water, then transferred to a 35.00 mL hermetic Teflon reactor and subjected to hydrothermal treatment at 110 °C for 24 h inside a flanged stainless steel hydrothermal reactor.After the hydrothermal treatment, the sample was centrifuged and washed twice with deionized water.Finally, the resulting precipitates were dried at 80 °C in an oven for 12 h.

Synthesis of TiO2/MWCNT Nanocomposite
The nanocomposite synthesis was adapted from the method Patel B.R. et al. proposed [34].In this modified method, depicted in Figure 2, 100.00 mg of TiO2 particles obtained in the previous step were combined with 50 mg of MWCNTs and added to 10.00 mL of 10 M NaOH solution in a microtube with a screw cap.The resulting mixture was sonicated for 30 min at 60 °C to ensure proper dispersion of the particles.Next, the mixture was Subsequently, 9.00 mL of a mixture of deionized water and solvent (3.00 mL H 2 O:6.00 mL of the ethanol-isopropanol solvent) were added dropwise to form TiO 2 particles through hydrolysis and condensation of titanium alkoxide species.After 2 h of magnetic stirring, the resulting colloidal suspension was centrifuged at 3500 rpm for 5 min and washed once with the solvent used in the reaction.
To crystallize the amorphous TiO 2 , the obtained material was resuspended in 32.00 mL of deionized water, then transferred to a 35.00 mL hermetic Teflon reactor and subjected to hydrothermal treatment at 110 • C for 24 h inside a flanged stainless steel hydrothermal reactor.After the hydrothermal treatment, the sample was centrifuged and washed twice with deionized water.Finally, the resulting precipitates were dried at 80 • C in an oven for 12 h.

Synthesis of TiO 2 /MWCNT Nanocomposite
The nanocomposite synthesis was adapted from the method Patel B.R. et al. proposed [34].In this modified method, depicted in Figure 2, 100.00 mg of TiO 2 particles obtained in the previous step were combined with 50 mg of MWCNTs and added to 10.00 mL of 10 M NaOH solution in a microtube with a screw cap.The resulting mixture was sonicated for 30 min at 60 • C to ensure proper dispersion of the particles.Next, the mixture was transferred to an autoclave container made of stainless steel, lined with Teflon material, and subjected to hydrothermal treatment at 130 • C for 24 h.
After cooling to room temperature, the nanocomposite was washed with 1 L of ultrapure water to remove impurities and unreacted species.The sample was then neutralized to a pH of 7.00 using a 0.50 M HCl solution.To ensure the complete removal of impurities, the nanocomposites were subjected to centrifugation at 5000 rpm with adding 0.50 L of ultrapure water.This washing step was repeated three times.
After cooling to room temperature, the nanocomposite was washed with 1 L of ultrapure water to remove impurities and unreacted species.The sample was then neutralized to a pH of 7.00 using a 0.50 M HCl solution.To ensure the complete removal of impurities, the nanocomposites were subjected to centrifugation at 5000 rpm with adding 0.50 L of ultrapure water.This washing step was repeated three times.
Finally, the nanocomposite was dried at 80 °C for 12 h to obtain a dry powder.The obtained material was named TiO2/MWCNT and was used for further studies.

Materials Characterization
X-ray diffraction analysis (XRD) has been carried out on the Shimadzu diffractometer (model 6100; Kyoto, Japan) using a Co Kα radiation source (λ = 1.788Å, 40 kV/30 mA).Customary conditions included a 2θ scan from 10° to 70°, a 0.02° angular step, and a 1 °/min scan speed.Data were converted to the Cu Kα wavelength using PowDLL software v 2.7 [35].The crystallographic phase identification was carried out by comparing the obtained patterns with JCPDS standards.
The samples' chemical structure and surface functional groups were analyzed using a Jasco FT/IR-4100 Fourier transform infrared spectrometer (FTIR) (Tokyo, Japan).The IR spectrum was recorded within the wavelength range of 4000 to 400 cm −1 .
The structure and surface morphology of the synthesized samples were analyzed using a scanning electron microscope (SEM), model JEOL JSM-6380LV (Tokyo, Japan), equipped with a Thermo Scientific Noran System SIX energy-dispersive X-ray spectrometer (EDS) (Waltham, MA, USA) operated at 20 kV.
Thermogravimetric measurements were conducted using a simultaneous thermal analyzer, the TGA-DSC Netzsch STA 449F3 Jupiter (Selb, Germany), in synthetic air from 30 °C to 900 °C at a heating rate of 10 °C/min.

Sensor Fabrication and Evaluation
A 3.00 mm diameter electrode was polished using 0.05 μm alumina slurry and thoroughly rinsed with distilled water to prepare the glassy carbon electrode for surface modification.A 10.00 μL dispersion of either MWCNTs or TiO2/MWCNT nanocomposite in ultrapure water, each at a concentration of 1.00 mg/mL, was then added to the GCE surface.The active materials were immobilized with Nafion ® -methanol (5.00% w/v) to create the MWCNT/Nafion/GCE and TiO2/MWCNT/Nafion/GCE sensors.One of the unique features of Nafion is its ability to facilitate proton transfer from its sulfonic groups to the Finally, the nanocomposite was dried at 80 • C for 12 h to obtain a dry powder.The obtained material was named TiO 2 /MWCNT and was used for further studies.The samples' chemical structure and surface functional groups were analyzed using a Jasco FT/IR-4100 Fourier transform infrared spectrometer (FTIR) (Tokyo, Japan).The IR spectrum was recorded within the wavelength range of 4000 to 400 cm −1 .

Materials Characterization
The structure and surface morphology of the synthesized samples were analyzed using a scanning electron microscope (SEM), model JEOL JSM-6380LV (Tokyo, Japan), equipped with a Thermo Scientific Noran System SIX energy-dispersive X-ray spectrometer (EDS) (Waltham, MA, USA) operated at 20 kV.
Thermogravimetric measurements were conducted using a simultaneous thermal analyzer, the TGA-DSC Netzsch STA 449F3 Jupiter (Selb, Germany), in synthetic air from 30 • C to 900 • C at a heating rate of 10 • C/min.

Sensor Fabrication and Evaluation
A 3.00 mm diameter electrode was polished using 0.05 µm alumina slurry and thoroughly rinsed with distilled water to prepare the glassy carbon electrode for surface modification.A 10.00 µL dispersion of either MWCNTs or TiO 2 /MWCNT nanocomposite in ultrapure water, each at a concentration of 1.00 mg/mL, was then added to the GCE surface.The active materials were immobilized with Nafion ® -methanol (5.00% w/v) to create the MWCNT/Nafion/GCE and TiO 2 /MWCNT/Nafion/GCE sensors.One of the unique features of Nafion is its ability to facilitate proton transfer from its sulfonic groups to the perfluorinated hydrophobic backbone, which results in the formation of a highly conductive medium for protons [10,36] to provide greater reticulation of the nanocomposites on the surface of the GCE.
Electrochemical sensing was conducted at room temperature using a three-electrode electrochemical cell (30.00 mL) consisting of the MWCNT/Nafion/GCE or TiO 2 /MWCNT/ Nafion/GCE as the working electrode, a Pt plate as the counter electrode, and Ag|AgCl as the reference electrode.The measurements were carried out using a portable bi-potentiostat/galvanostat µ-Autolab Type III (Metrohm Autolab, Utrecht, The Netherlands) controlled by Nova 2.1 software.
The electrochemical sensing was performed in different H 2 O 2 concentrations in a 0.10 M Britton-Robinson (B-R) buffer at pH 7.00 under constant magnetic stirring.After each triplicate, the solutions were homogenized by stirring for 30 s, and any electrogenerated products were removed from the electrode surface.For the addition-recovery experiments, standard solutions of H 2 O 2 were prepared using 0.01 M (mol•L −1 ) stock solutions in ultrapure water (R ≥ 18.2 MΩ cm).

Characterization of MWCNT and TiO 2 /MWCNTs Nanocomposite
Figure 3 shows the XRD diffraction patterns of both pristine MWCNT and TiO 2 /MWCNT nanocomposite.Although some peaks present low intensity, smoothing has been applied to distinguish them better.perfluorinated hydrophobic backbone, which results in the formation of a highly conductive medium for protons [10,36] to provide greater reticulation of the nanocomposites on the surface of the GCE.
Electrochemical sensing was conducted at room temperature using a three-electrode electrochemical cell (30.00 mL) consisting of the MWCNT/Nafion/GCE or TiO2/MWCNT/Nafion/GCE as the working electrode, a Pt plate as the counter electrode, and Ag|AgCl as the reference electrode.The measurements were carried out using a portable bi-potentiostat/galvanostat μ-Autolab Type III (Metrohm Autolab, Utrecht, The Netherlands) controlled by Nova 2.1 software.
The electrochemical sensing was performed in different H2O2 concentrations in a 0.10 M Britton-Robinson (B-R) buffer at pH 7.00 under constant magnetic stirring.After each triplicate, the solutions were homogenized by stirring for 30 s, and any electrogenerated products were removed from the electrode surface.For the addition-recovery experiments, standard solutions of H2O2 were prepared using 0.01 M (mol•L −1 ) stock solutions in ultrapure water (R ≥ 18.2 MΩ cm).

Characterization of MWCNT and TiO2/MWCNTs Nanocomposite
Figure 3 shows the XRD diffraction patterns of both pristine MWCNT and TiO2/MWCNT nanocomposite.Although some peaks present low intensity, smoothing has been applied to distinguish them better.The XRD patterns of pristine MWCNTs show diffraction peaks of graphite structures at 2θ = 25.8 • and 43.5 • (JCPDS card 41-1487) corresponding to reflections from crystallographic planes (002), the spacing between adjacent graphite layers, and ordering within the plane (100), respectively.
In addition to the diffraction peaks related to pristine MWCNTs, the MWCNT/TiO 2 nanocomposite exhibits a diffraction peak at 2θ = 48.2• related to the (200) plane, indicating the formation of the anatase phase of TiO 2 (JCPDS card no.21-1272).The significant broadening of the peak indicates the nanometric characteristic of the TiO 2 particles.Thus, the hydrothermal treatment employed was effective, as it promoted the crystallization of amorphous titania into the anatase phase.
An approximate measure of the crystallite size of the TiO 2 phase in the sample was determined using the well-known Scherrer equation [37], which is based on the full width at half maximum (FWHM) Å obtained by a Gaussian function, diffraction angle θ and the wavelength λ associated with Cu Kα radiation.The average crystallite size of anatase (200) was found to be 5.50 nm based on the broadening of its diffraction peak, where the FWHM is 1.63 Å.
The FTIR spectra of pristine MWCNTs, bare TiO 2 particles, and TiO 2 /MWCNT nanocomposite are shown in Figure 4.
the hydrothermal treatment employed was effective, as it promoted the crystallization of amorphous titania into the anatase phase.
An approximate measure of the crystallite size of the TiO2 phase in the sample was determined using the well-known Scherrer equation [37], which is based on the full width at half maximum (FWHM) Å obtained by a Gaussian function, diffraction angle θ and the wavelength λ associated with Cu Kα radiation.The average crystallite size of anatase (200) was found to be 5.50 nm based on the broadening of its diffraction peak, where the FWHM is 1.63 Å.
The FTIR spectra of pristine MWCNTs, bare TiO2 particles, and TiO2/MWCNT nanocomposite are shown in Figure 4.The FTIR spectra of all samples showed a broad absorption band in the range of 3000 to 3500 cm −1 , centered at 3219 cm −1 , attributed to the O-H stretching vibration and the surface adsorbed water [38].In the MWCNTs' FTIR spectrum, distinct peaks were observed at 1539, 1728, and 3219 cm −1 .The peak at 1539 cm −1 confirmed the presence of graphitic carbon bonds (C=C stretching vibration), while the peak at 1728 cm −1 indicated the presence of carbonyl (C=O) functional groups [39,40].
The FTIR spectrum of bare TiO2 particles shows a broad band of 1000 to 400 cm −1 , which can be attributed to various stretching vibrations, including Ti-O and O-Ti-O bonds [41].However, in the FTIR spectrum of the TiO2/MWCNT nanocomposite, in addition to the Ti-O and O-Ti-O bonds, the presence of Ti-O-C and Ti-O-C=O bonds is observed, indicating an interaction between TiO2 particles and the MWCNTs [42].Furthermore, the anatase titania phase, as revealed by XRD analysis, contributes to the observed band [43,44].Both the nanocomposite and bare TiO2 particles exhibit a distinct band ~1395 cm −1 corresponding to TiO2 lattice vibrations [45,46].In addition, the FTIR spectrum of the TiO2/MWCNT nanocomposite also displays a peak at 1633 cm −1 , representing the deformative vibration of the Ti-OH stretching mode and the OH stretch of adsorbed water.The FTIR spectra of all samples showed a broad absorption band in the range of 3000 to 3500 cm −1 , centered at 3219 cm −1 , attributed to the O-H stretching vibration and the surface adsorbed water [38].In the MWCNTs' FTIR spectrum, distinct peaks were observed at 1539, 1728, and 3219 cm −1 .The peak at 1539 cm −1 confirmed the presence of graphitic carbon bonds (C=C stretching vibration), while the peak at 1728 cm −1 indicated the presence of carbonyl (C=O) functional groups [39,40].
The FTIR spectrum of bare TiO 2 particles shows a broad band of 1000 to 400 cm −1 , which can be attributed to various stretching vibrations, including Ti-O and O-Ti-O bonds [41].However, in the FTIR spectrum of the TiO 2 /MWCNT nanocomposite, in addition to the Ti-O and O-Ti-O bonds, the presence of Ti-O-C and Ti-O-C=O bonds is observed, indicating an interaction between TiO 2 particles and the MWCNTs [42].Furthermore, the anatase titania phase, as revealed by XRD analysis, contributes to the observed band [43,44].Both the nanocomposite and bare TiO 2 particles exhibit a distinct band ~1395 cm −1 corresponding to TiO 2 lattice vibrations [45,46].In addition, the FTIR spectrum of the TiO 2 /MWCNT nanocomposite also displays a peak at 1633 cm −1 , representing the deformative vibration of the Ti-OH stretching mode and the OH stretch of adsorbed water.Moreover, a band at 1100 cm −1 has been attributed to alkoxy C-O stretching vibrations within the nanocomposite [47].
Thermogravimetric analysis was used to investigate the thermal stability of the studied materials.According to Figure 5, it can be observed that the MWCNT shows a mass loss caused by the oxidation of the nanotubes with a peak temperature at 575 • C, which agrees with values reported in the literature ranging from 550-650 • C [48].The stability was achieved with a residue of 3.50%, resulting from the remaining catalysts from the synthesis of the nanotubes (MWCNT), as reported by the supplier of this material.
Thermogravimetric analysis was used to investigate the thermal stability of the studied materials.According to Figure 5, it can be observed that the MWCNT shows a mass loss caused by the oxidation of the nanotubes with a peak temperature at 575 °C, which agrees with values reported in the literature ranging from 550-650 °C [48].The stability was achieved with a residue of 3.50%, resulting from the remaining catalysts from the synthesis of the nanotubes (MWCNT), as reported by the supplier of this material.The MWCNT/TiO2 nanocomposite exhibited a mass loss of 5.00% up to 100 °C, which can be attributed to adsorbed water on the material's surface-this mass loss results from the desorption of the water molecules from the nanocomposite.Between the temperature range of 100 and 450 °C, a further mass loss of approximately 8.00% was observed.This additional mass loss is believed to be caused by the decomposition of oxygenated groups that were incorporated into the MWCNTs during the synthesis process of the nanocomposite [49].The presence of these oxygenated groups provides evidence for the bonding mechanism between TiO2 and MWCNT, suggesting that the bonding occurs through the involvement of these functional groups.This bonding interaction between TiO2 and MWCNT is further supported by the presence of Ti-O-C and Ti-O-C=O bonds, as observed in FTIR analysis.
Furthermore, it was observed that the temperature range at which significant mass loss occurs because of the oxidation of nanotubes is between 500-690 °C.This can be easily seen in the DTG curve.The constant levels observed in the TGA curve after 690 °C indicate that the nanotubes underwent complete oxidation.The residue reflects the percentage of TiO2 present in the nanocomposite: approximately 41.50%.
The morphology of pristine MWCNT and MWCNT/TiO2 nanocomposite samples were evaluated using SEM. Figure 6a shows that the pristine MWCNT has a smooth surface with entangled tube bundles.In Figure 6b-d, it can be observed that the MWCNT/TiO2 has morphology like the undecorated MWCNTs, i.e., no particles that could be attributed to TiO2 were marked.The MWCNT/TiO 2 nanocomposite exhibited a mass loss of 5.00% up to 100 • C, which can be attributed to adsorbed water on the material's surface-this mass loss results from the desorption of the water molecules from the nanocomposite.Between the temperature range of 100 and 450 • C, a further mass loss of approximately 8.00% was observed.This additional mass loss is believed to be caused by the decomposition of oxygenated groups that were incorporated into the MWCNTs during the synthesis process of the nanocomposite [49].The presence of these oxygenated groups provides evidence for the bonding mechanism between TiO 2 and MWCNT, suggesting that the bonding occurs through the involvement of these functional groups.This bonding interaction between TiO 2 and MWCNT is further supported by the presence of Ti-O-C and Ti-O-C=O bonds, as observed in FTIR analysis.
Furthermore, it was observed that the temperature range at which significant mass loss occurs because of the oxidation of nanotubes is between 500-690 • C.This can be easily seen in the DTG curve.The constant levels observed in the TGA curve after 690 • C indicate that the nanotubes underwent complete oxidation.The residue reflects the percentage of TiO 2 present in the nanocomposite: approximately 41.50%.
The morphology of pristine MWCNT and MWCNT/TiO 2 nanocomposite samples were evaluated using SEM. Figure 6a shows that the pristine MWCNT has a smooth surface with entangled tube bundles.In Figure 6b-d, it can be observed that the MWCNT/TiO 2 has morphology like the undecorated MWCNTs, i.e., no particles that could be attributed to TiO 2 were marked.
The results from the EDS analysis, as shown in Figure 7, support the presence of titanium in the nanocomposite and its successful integration into the carbon nanotube matrix.These findings are consistent with the XRD and FTIR analysis results.

Electrochemical Behavior of GCE, MWCNT/Nafion, and TiO 2 /MWCNT/Nafion-Modified GCEs
The electrocatalytic activity of modified working electrodes was examined by measuring the cyclic voltammograms (CVs) in 0.50 M H 2 SO 4 within the potential range of −0.5 to 1.5 V vs. Ag|AgCl at a scan rate of 50 mVs −1 .As depicted in Figure 8a, the unmodified GCE was determined to be electrochemically inactive under these conditions, which aligns with the earlier observations reported by Benck et al. [50].The CV of the bare CGE displayed no redox pair, resulting in a narrow and almost flat line in the graph.This implies that no electrochemical reactions were taking place on the surface of the CGE electrode under the specified conditions.

Electrochemical Behavior of GCE, MWCNT/Nafion, and TiO2/MWCNT/Nafion-Modified GCEs
The electrocatalytic activity of modified working electrodes was examined by measuring the cyclic voltammograms (CVs) in 0.50 M H2SO4 within the potential range of −0.5 to 1.5 V vs. Ag|AgCl at a scan rate of 50 mVs −1 .As depicted in Figure 8a, the unmodified GCE was determined to be electrochemically inactive under these conditions, which aligns with the earlier observations reported by Benck et al. [50].The CV of the bare CGE displayed no redox pair, resulting in a narrow and almost flat line in the graph.This implies that no electrochemical reactions were taking place on the surface of the CGE electrode under the specified conditions.However, in Figure 8a, the modification of the CGE electrode with MWCNT altered the electrochemical profile of the surface, broadening the area of the voltammogram, as the carbon nanotubes provided a surface and electroactive increment.In addition, two peaks, A1 and C1, are observed, which correlate with the redox reactions of metallic iron resulting from the catalyst used to synthesize MWCNTs [51,52].Notably, the potential scan toward more negative values for the GCE/MWCNT/TiO2 electrode gives rise to redox current peaks in the potential region between −0.5 and 0.0 V, which may be associated with the oxidation/reduction in Ti ions [53][54][55][56].
Moreover, the presence of oxygenated functional groups, such as hydroxyls (OH) and carbonyls (C=O), in the TiO2/MWCNT nanocomposite plays a significant role in its electrochemical behavior.These functional groups contribute to increased adsorption capacity for species and provide additional active sites for the adsorption of molecules and ions.Consequently, electrochemical reactions, including the oxidation of H2O2, can take place more efficiently.This makes the modified GCE electrode an up-and-coming candidate for H2O2-sensing applications where accurate and sensitive detection of H2O2 is required [57][58][59].

Evaluation of TiO2/MWCNT/Nafion/GCE Electrode as an Electrochemical Sensor for H2O2 Determination
The electrocatalytic activity of the TiO2/MWCNT/Nafion/GCE sensor toward the oxidation of H2O2 was investigated by CV. Figure 9a shows the cyclic voltammograms of the electrode in a Britton-Robinson (B-R) buffer solution at pH 7, under a potential scan range  However, in Figure 8a, the modification of the CGE electrode with MWCNT altered the electrochemical profile of the surface, broadening the area of the voltammogram, as the carbon nanotubes provided a surface and electroactive increment.In addition, two peaks, A1 and C1, are observed, which correlate with the redox reactions of metallic iron resulting from the catalyst used to synthesize MWCNTs [51,52].Notably, the potential scan toward more negative values for the GCE/MWCNT/TiO 2 electrode gives rise to redox current peaks in the potential region between −0.5 and 0.0 V, which may be associated with the oxidation/reduction in Ti ions [53][54][55][56].
Moreover, the presence of oxygenated functional groups, such as hydroxyls (OH) and carbonyls (C=O), in the TiO 2 /MWCNT nanocomposite plays a significant role in its electrochemical behavior.These functional groups contribute to increased adsorption capacity for species and provide additional active sites for the adsorption of molecules and ions.Consequently, electrochemical reactions, including the oxidation of H 2 O 2 , can take place more efficiently.This makes the modified GCE electrode an up-and-coming candidate for H 2 O 2 -sensing applications where accurate and sensitive detection of H 2 O 2 is required [57][58][59].

Evaluation of TiO 2 /MWCNT/Nafion/GCE Electrode as an Electrochemical Sensor for H 2 O 2 Determination
The electrocatalytic activity of the TiO 2 /MWCNT/Nafion/GCE sensor toward the oxidation of H 2 O 2 was investigated by CV. Figure 9a shows the cyclic voltammograms of the electrode in a Britton-Robinson (B-R) buffer solution at pH 7, under a potential scan range of −0.2 V to 1.0 V, in the absence of and after successive additions of hydrogen peroxide (from 0 up to 120 µM) to the cell solution, and the corresponding calibration plot.
The cyclic voltammograms demonstrated the direct proportionality of the peak current to H 2 O 2 concentration within 14.00-120.00µM.The linear regression equation illustrated Ip (µA) = 1.0145x − 0.0346, (R 2 = 0.9985) (Figure 9b).The detection limits (LD) and quantification limits (LQ) were calculated according to the recommendations of the IUPAC as three times the standard deviation of the blank signal-Britton-Robinson buffer-(σB) divided by the slope of the calibration curve (m): LD = 3 σB/m.The LQs were calculated similarly, with 10 replacing 3 in each equation, i.e., LQ = 10 σB/m.Therefore, the LD and LQ limits obtained were 4.00 µM and 14.00 µM, respectively.The cyclic voltammograms demonstrated the direct proportionality of the peak current to H2O2 concentration within 14.00-120.00μM.The linear regression equation illustrated Ip (μА) = 1.0145x − 0.0346, (R 2 = 0.9985) (Figure 9b).The detection limits (LD) and quantification limits (LQ) were calculated according to the recommendations of the IU-PAC as three times the standard deviation of the blank signal-Britton-Robinson buffer-(σB) divided by the slope of the calibration curve (m): LD = 3 σB/m.The LQs were calculated similarly, with 10 replacing 3 in each equation, i.e., LQ = 10 σB/m.Therefore, the LD and LQ limits obtained were 4.00 μM and 14.00 μM, respectively.
The fabricated sensor's accuracy was evaluated using the standard addition method, in distilled deionized water, at three different concentration levels of H2O2 (20.00, 40.00, and 60.00 μM) Britton-Robinson buffer solution at a pH of 7.00, using cyclic voltammetry.
The results are presented in Table 1, demonstrating a range of variation between 67.30% and 101.90%.Our findings indicate that this analytical method is only sufficiently accurate for concentrations greater than 20.00 μM.A comparison between the TiO2/MWCNT/Nafion/GCE electrode and other GCEmodified electrodes reported in the literature is presented in Table 2. Our results align with those of other studies, indicating that this electrode coating is suitable for detecting H2O2.Furthermore, it is worth noting that the TiO2/MWCNT/Nafion/GCE electrode can be produced with fewer laborious steps than in some of the previous works.The fabricated sensor's accuracy was evaluated using the standard addition method, in distilled deionized water, at three different concentration levels of H 2 O 2 (20.00, 40.00, and 60.00 µM) Britton-Robinson buffer solution at a pH of 7.00, using cyclic voltammetry.
The results are presented in Table 1, demonstrating a range of variation between 67.30% and 101.90%.Our findings indicate that this analytical method is only sufficiently accurate for concentrations greater than 20.00 µM.A comparison between the TiO 2 /MWCNT/Nafion/GCE electrode and other GCEmodified electrodes reported in the literature is presented in Table 2. Our results align with those of other studies, indicating that this electrode coating is suitable for detecting H 2 O 2 .Furthermore, it is worth noting that the TiO 2 /MWCNT/Nafion/GCE electrode can be produced with fewer laborious steps than in some of the previous works.
Considering that hydrogen peroxide concentrations typically range from micromolar for in vivo conditions and residual levels in foodstuffs and drinking water to tens of millimolar for bleaching applications and molar for waste treatment applications [67], the TiO 2 /MWCNT/Nafion/GCE has the potential to determine H 2 O 2 levels in water.

Conclusions
In summary, a novel H 2 O 2 voltammetric sensor has been developed by modifying a glassy carbon electrode (GCE) with a TiO 2 /MWCNT/Nafion nanocomposite synthesized using a facile hydrothermal route.Incorporating semiconducting TiO 2 nanoparticles and highly conducting MWCNTs synergistically enhanced the sensor's performance toward H 2 O 2 detection, surpassing the performance of both bare GCE and MWCNT/Nafionmodified GCE electrodes.The proposed modified electrode successfully detected and determined H 2 O 2 concentrations in water ranging from 14.00-120.00µM, with a detection limit of 4.00 µM.
These findings represent a significant advancement in the design and fabrication of highly efficient H 2 O 2 -based sensor devices and have important implications for various applications such as water treatment, food safety, and biomedical diagnostics.Using the TiO 2 /MWCNT/Nafion nanocomposite as a sensing material holds great promise for developing cost-effective, reliable, and sensitive H 2 O 2 sensors with a wide range of detection capabilities.
Further research could explore the potential of this novel sensor platform for other analytes and environmental conditions, paving the way for the development of nextgeneration sensing technologies.However, it is essential to note that extensive testing and validation of the sensor's performance under different conditions will be necessary to ensure its practical applicability before commercialization.

Figure 1 .
Figure 1.Diagram of preparation of TiO2 nanoparticles by combined sol-gel and hydrothermal methods.

Figure 1 .
Figure 1.Diagram of preparation of TiO 2 nanoparticles by combined sol-gel and hydrothermal methods.

Figure 2 .
Figure 2. Schematic diagram of the preparation of TiO2/MWCNT nanocomposite by hydrothermal method.

Figure 2 .
Figure 2. Schematic diagram of the preparation of TiO 2 /MWCNT nanocomposite by hydrothermal method.

X
-ray diffraction analysis (XRD) has been carried out on the Shimadzu diffractometer (model 6100; Kyoto, Japan) using a Co Kα radiation source (λ = 1.788Å, 40 kV/30 mA).Customary conditions included a 2θ scan from 10 • to 70 • , a 0.02 • angular step, and a 1 • /min scan speed.Data were converted to the Cu Kα wavelength using PowDLL software v 2.7 [35].The crystallographic phase identification was carried out by comparing the obtained patterns with JCPDS standards.

Figure 3 .Figure 3 .
Figure 3. XRD patterns of pristine MWCNT and TiO2/MWCNT nanocomposite.The XRD patterns of pristine MWCNTs show diffraction peaks of graphite structures at 2θ = 25.8° and 43.5° (JCPDS card 41-1487) corresponding to reflections from crystallographic planes (002), the spacing between adjacent graphite layers, and ordering within the plane (100), respectively.In addition to the diffraction peaks related to pristine MWCNTs, the MWCNT/TiO2 nanocomposite exhibits a diffraction peak at 2θ = 48.2°related to the (200) plane, indicating the formation of the anatase phase of TiO2 (JCPDS card no.21-1272).The significant broadening of the peak indicates the nanometric characteristic of the TiO2 particles.Thus, Figure 3. XRD patterns of pristine MWCNT and TiO 2 /MWCNT nanocomposite.

Figure 7 .
Figure 7. SEM image of the MWCNT/TiO2 nanocomposite (a) and corresponding EDS elemental mapping with separate maps shown for (b) C, (c) Ti, and (d) O.

Figure 7 .
Figure 7. SEM image of the MWCNT/TiO 2 nanocomposite (a) and corresponding EDS elemental mapping with separate maps shown for (b) C, (c) Ti, and (d) O.

Sensors 2023 ,
23,  x FOR PEER REVIEW 11 of 15 of −0.2 V to 1.0 V, in the absence of and after successive additions of hydrogen peroxide (from 0 up to 120 μM) to the cell solution, and the corresponding calibration plot.

Table 1 .
Results for the recovery study in distilled-deionized water at three concentration levels for H2O2 in Britton-Robinson buffer solution at pH 7.00.

Table 1 .
Results for the recovery study in distilled-deionized water at three concentration levels for H 2 O 2 in Britton-Robinson buffer solution at pH 7.00.